From 8911c9595b9d553100951ff2653464ef5868b81d Mon Sep 17 00:00:00 2001 From: Benedict Date: Mon, 15 Aug 2016 12:22:31 +0200 Subject: set4, completed challenge 25 --- set4/25.txt | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ set4/Makefile | 9 +++++++++ set4/task25.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 126 insertions(+) create mode 100644 set4/25.txt create mode 100644 set4/Makefile create mode 100644 set4/task25.c (limited to 'set4') diff --git a/set4/25.txt b/set4/25.txt new file mode 100644 index 0000000..c02ff8a --- /dev/null +++ b/set4/25.txt @@ -0,0 +1,64 @@ +CRIwqt4+szDbqkNY+I0qbDe3LQz0wiw0SuxBQtAM5TDdMbjCMD/venUDW9BL +PEXODbk6a48oMbAY6DDZsuLbc0uR9cp9hQ0QQGATyyCESq2NSsvhx5zKlLtz +dsnfK5ED5srKjK7Fz4Q38/ttd+stL/9WnDzlJvAo7WBsjI5YJc2gmAYayNfm +CW2lhZE/ZLG0CBD2aPw0W417QYb4cAIOW92jYRiJ4PTsBBHDe8o4JwqaUac6 +rqdi833kbyAOV/Y2RMbN0oDb9Rq8uRHvbrqQJaJieaswEtMkgUt3P5Ttgeh7 +J+hE6TR0uHot8WzHyAKNbUWHoi/5zcRCUipvVOYLoBZXlNu4qnwoCZRSBgvC 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$(TARGETS) diff --git a/set4/task25.c b/set4/task25.c new file mode 100644 index 0000000..7a29bf9 --- /dev/null +++ b/set4/task25.c @@ -0,0 +1,53 @@ +#include "../lib/lib.h" +#include "../lib/lib2.h" +#include "../lib/lib3.h" +#include "../lib/lib4.h" +#include + +/** + * Assume same key AND same nonce, this leads to the same keystream + * every time. Than it is easy to discover the keystream with the edit + * function. Simple XOR cihertext agianst out inserted text. + * + * If the nonce change for each edit call, the above won't work. We have for + * every edit a new keystream, that makes it harder. + * + * + **/ + +int main() +{ + generate_random_bytes(key, 16); + generate_random_bytes(nonce, 16); + + char *file_content; + int file_length = read_base64_file("25.txt", &file_content); + char *base64_decoded = malloc(file_length); + + int base64_decoded_length = decode_base64(file_content, base64_decoded); + // the file ist AES-ECB encrypted + char *plain_tmp = malloc(base64_decoded_length); + aes_ecb(base64_decoded, base64_decoded_length, plain_tmp, "YELLOW SUBMARINE", + 16, 0); + char *ciphertext = malloc(base64_decoded_length); + + aes_ctr(plain_tmp, base64_decoded_length, ciphertext, key, nonce); + + // start the attack + // recover 4-byte-keystream in each iteration + char *org_ciphertext = malloc(base64_decoded_length); + memcpy(org_ciphertext, ciphertext, base64_decoded_length); + int i, j; + char keystream[16]; + char *plaintext = malloc(base64_decoded_length); + char newtext[16] = "ABABABABABABABAB"; + for(i=0;i<(base64_decoded_length/16);i++) { + aes_ctr_edit(ciphertext, base64_decoded_length, (i*16), newtext); + for(j=0;j<16;j++) { + keystream[j] = newtext[j] ^ ciphertext[i*16+j]; + plaintext[i*16+j] = keystream[j] ^ org_ciphertext[i*16+j]; + } + } + + printf("Recovered plaintext: %s\n", plaintext); +} -- cgit v1.2.3-70-g09d2